mac caryotakis 1106

8/14/2019 Mac Caryotakis 1106

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XP3-1/XP3-2 TEST RESULTS

There were several factors contributing to the poor performance of these DFM PPM-focused 75-MW klystrons:

1. Although the smaller DFM gun ceramic had been tested in a beam tester with

3.2 s pulses at full voltage, it had been designed for only 1.5 s (before the pulselength was doubled). The gun in XP3 2 proved to be not sufficiently sturdy forthe longer pulse length. The ceramic in that tube was damaged early at test;subsequently, XP3-2 was limited in beam voltage and peak power.

2. The output cavities in these klystrons have a potential trapped mode atapproximately 11.7 MHz. This results from the mismatch presented to the outputcavities by the mode converters ahead of the windows. We suppress it bycoupling resonant loss cavities between the output cavity and the 2 windows.These cavities are designed so that, when located correctly on the waveguide,they match the window and mode converter to the output cavity at 11.7 GHz. Thetrapped mode is then loaded over a narrow frequency range. The potential for this

oscillation, however, is exacerbated further if the 3 penultimate klystroncavities, which are tuned well above the operating frequency in order to improve rfbeam bunching, and are therefore are tuned close to the 11. 7 GHz frequency. InXP3-1 the resonant loss cavity was placed at an incorrect location. Also, in bothklystrons, the penultimate cavities were tuned too close to 11.7 GHz.

3. The 2 polepieces in the gun electromagnet, which is used to launch the beam intothe PPM stack, were split in both klystrons, causing transverse magnetic fields toexist in the vicinity of the cathode. These fields were an order of magnitude

higher that we specify for our PPM stacks (about 1% vs. 0.2% of the axial field).This may have caused cork-screwing in the beams of both klystrons. Evidenceof that effect may have been the reduced available range of electromagnet currentin adjusting the size of the klystron beam for optimum output power.

4. The same comment applies to potential transverse magnetic field in the PPMclamshells, which was also measured to be as high as 0.8%. Although theclamshells had been tested for such fields, the tests (which must measure a few

transverse gauss in the presence of an axial field of 3000 Gauss) werequestionable. They have been repeated and the results are shown in Fig. 10.Unfortunately no analytical or simulated predictions have existed on the limits fortransverse fields in PPM stacks. This is being remedied by using a recentlyavailable 3D code.


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SLAC PPM KLYSTRON DEVELOPMENT

50 XP

1996-99

1

XP 1

1997-99

2

XP 3 Diode

2001

XP 3-1 (DFM)

2002

XP 3-2 (DFM)

2002

XP 4

2003Power Output

Design

Attained

50

50 60

75

70 - 90

Beam Tester 75

50 70

75

30 40

75

Pulse Length

Design

Attained

1.2

2.4

1.5

3.0

1.5

3.2

3.2

3.2

3.2

2.8

3.2

PRF

DesignAttained

60120

6060

120120

120120

12060

120

Best results

(simultaneous)

Po-s-Hz-%50-2.4-120-57 79-2.8-1-62 1wk 490kV

3.2s, 120 pps

70-0.3-120-55

50-3.2-120-39

40-0.5-120-31

30-2.8-120-24

Polepieces Brazed to

form the drift

tube

Brazed to

s.s. tube

Brazed to

s.s. tube

Clamp on Clamp on Brazed to

s.s. tube

Gun Seal Large (11.5)

tapered

Large,

tapered

Small (9),

straight

Small, straight Small, straight Large,

tapered

Gun Stability3 Good Good

LC

Good

LC

Good

LC

Good

Gun

Breakdown4

LC

Beam Transport

(no rf) Excellent

99.9% Fair 99%

4

Good 99.5%

Good 99.5%

but a narrow

coil range

Good 99.5%

Output Stability Excellent Excellent N/A 11.7GHz

Output

Oscillation4

Good

Average Power

Design

Attained

3.6 kW

14.4 kW

0.1kW0.2 kW

DC

23 kW

49 kW

29 kW

19.2 kW

29 kW

10 kW

29 kW

1Tube originally tested in 1997, but was retested in 1999 for longer pulse length and higher PRF.

Perveance: 0.6. All subsequent klystrons built with K=0.75. Tube was rebuilt once in order to coat the

polepiece IDs and eliminate multipactor.


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5

Modifications incorporated to the rebuilt XP 3-2

Cavity tuning changed for optimum efficiency and oscillationavoidance.

Resonant loss cavity placed at correct location.

Gun coil split polepiece remade without split.

Gun from XP 3-1 will be used in place of the XP 3-2 gun,which was damaged from arcing.

And, most important, no more 3.2 sec pulses to triggeroscillations or cause gun breakdowns


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7

110 tunings investigated: Saturated Gain and Pout vs. Sum of

the frequencies of cavities 5, 6 and 7. Multiple solutionsfound which avoid high gradients and pre-determined

unstable frequencies, and also give good BW, gain and power

out.

45

50

55

60

65

70

75

80

34800 35000 35200 35400 35600 35800 36000

Sum of F's (MHz)

Gain,P

wr

Pout

Gain

XP3 XP3 Rebuild

11700 X 3


8/11

8

Old Loss cavity location S11 vs. new location shows better than 10dB improvement in reflection from

mode converter-load assy at 11.7GHz (good match).

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1.3 1.5 1.7 1.9 2.1 2.3 2.5

Loss Cavity Location (inches)

S11

(dB)

Meas. Load A: TS1794-4

Calc. Load A

New Location

Old


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Transverse B fields at gap centers (minimum points on curve below) average > 0.2% for the lower XP3-1 clamp-on

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

-500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0

mm

10

Bt/Bz(RMS)


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